Iboc compatible superposition modulation by independent modulators utilizing clipping noise from peak-to-average power reduction

ABSTRACT

According to an aspect of the present invention, there is provided a method for providing additional bandwidth to receivers that can decode a higher modulation comprising trading a peak-to-average power ratio (PAPR) reduction induced constellation noise of all or a subset of in-band on-channel (IBOC) carriers within an orthogonal frequency division multiplexing (ODFM) waveform with data carrying superposition modulation.

PRIORITY CLAIM

This application claims the priority benefit of Canadian PatentApplication Serial Number 3070530, filed Jan. 30, 2020, the disclosureof which is incorporated herein by reference in its entirety.

BACKGROUND

The In-Band On-Channel (IBOC) standard employed by HD Radio®, as well asother digital broadcast standards like Digital Radio Mondiale (DRM),China Digital Radio (CDR), or Digital Audio Broadcasting (DAB), employOrthogonal Frequency Division Multiplexing (OFDM) to achieve high datathroughput with high spectral efficiency. The addition of a multitude ofOFDM carriers produces a signal envelope with high peak-to-average powerratios (PAPR) (Schmid, 2009). Broadcast amplifiers typically must bebacked-off in order to accommodate these signal peaks, requiring largertransmitters and reducing efficiency. This problem is well known in theindustry and many approaches have been explored to reduce the PAPR ofthe pure OFDM signal. The approaches taught by (Shelswell, 1992) and(Anjali Shastri, 1999) maintain, control, and limit noise within thesignal constellation introduced by clipping the signal peak with itapplied to a Quadrature Phase Shift Keyed (QPSK) signal constellation(other constellation types are possible). The aim of these algorithms istypically to improve signal quality by correcting the phase and/oramplitude of the constellation to move all points away from the decisionboundary. Many digital transmission standards impose limitations on theModulation Error Ratio (MER) in order to guarantee a minimum acceptablesignal quality emitted from the broadcast transmitter. The NationalRadio Systems Committee (NRSC) defined such a specification for IBOCsignal quality in (National Radio Systems Committee (NRSC), 2011).

SUMMARY OF THE INVENTION

The invention has a number of aspects that may be exploited individuallyor in combination.

According to an aspect of the present invention, there is provided amethod for providing additional bandwidth to receivers that can decode ahigher modulation comprising trading a peak-to-average power ratio(PAPR) reduction induced constellation noise of all or a subset ofin-band on-channel (IBOC) carriers within an orthogonal frequencydivision multiplexing (ODFM) waveform with data carrying superpositionmodulation.

In one embodiment, the IBOC carrier is Quadrature Phase Shift Keyed(QPSK) modulated.

In another embodiment, the superposition modulation is hierarchicalmodulation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings wherein:

FIG. 1 is a graphical representation of IBOC signal constellation withPAPR reduction induced clipping noise and superpositioned QPSKmodulation for a subset of IBOC carriers. All IBOC carriers exhibitroughly 14 dB MER that is within IBOC quality specifications;

FIG. 2 shows superposition modulation of 5 outer most IBOC partitions(left) with PAR reduction noise on remaining partitions (right),reference carriers are unaffected;

FIG. 3 shows IBOC with superpositioned QPSK modulation on a runningtransmitter with increased PAPR;

FIG. 4 shows binary phase shift keying (BSPK) super positioned on IBOCcarriers; and

FIG. 5 shows dual independent modulators for super position modulation.

DETAILED DESCRIPTION

Throughout the following description, specific details are set forth inorder to provide a more thorough understanding of the invention.However, the invention may be practiced without these particulars. Inother instances, well known elements may not have been shown ordescribed in detail to avoid unnecessarily obscuring the disclosure.Accordingly, the description and drawings are to be regarded asillustrative, rather than restrictive. Immaterial modifications may bemade to the embodiments described herein without departing from what iscovered by the claims.

The method described herein involves trading the peak-to-average powerratios (PAPR) reduction induced constellation noise of all or a subsetof the In-Band On-Channel (IBOC) carriers (Quadrature Phase Shift Keyed(QPSK) modulated or other) within the Orthogonal Frequency DivisionMultiplexing (OFDM) waveform with data carrying superpositionmodulation, also known as hierarchical modulation, providing additionalbandwidth to receivers that can decode the higher order modulation.Shown in FIG. 1 as red circles. Standard receivers will ignore the addedmodulation and consider it no different than other constellation noiseand will not be negatively affected, provided the added modulation canalso comply with the required signal quality specification. Existingbroadcast transmitters already utilize this portion of thespectrum/constellation for PAPR reduction while complying with allestablished signal quality standards and specifications. These standardsdo not specify the intended purpose of the constellation noise;therefore, it can be repurposed for superposition modulation. As such,no special ruling should be required by a spectrum regulator, such asthe Federal Communications Commission (FCC). A broadcaster can unlockthis spectrum asset without causing undue interference nor harm to theirown main signal transmission.

Methods to provide super positioned modulation are well-known in theindustry. The method described herein is to integrate this type ofmodulation within the PAPR reduction algorithm in such a way as toprovide a flexible trade-off between data capacity and PAPR affectingtransmitter power performance, while complying with established signalquality specifications.

FIG. 2 shows the demodulated IBOC signal constellation of two sidebandscomposed of 11 frequency partitions (MP2 mode) per sideband, of whichthe outer 5 ones are allocated to superposition modulation (QPSK in thisexample) and the inner 6 are allocated to normal IBOC modulationincluding PAPR reduction noise. Many other combinations are possibleranging from all partitions being used for superposition modulation tosingular partitions to individual carriers in any combination. Referencecarriers could be used but are not typically employed in order to ensurebest possible signal tracking and channel equalization by the receiver.A digital receiver that is capable of decoding both the primary andsecondary modulation can utilize the same symbol tracking, equalizationand other signal conditioning for both sets of modulation. If thesecondary modulation is only on a single sideband, the receiver onlyneeds to decode that particular sideband leading to receiver costimprovements as a lower bandwidth front end would be required.Information about the secondary modulation (carrier location, etc . . .. ) can be conveyed via data services on the primary modulation or viceversa.

The broadcaster has flexibility in how much of the signal to dedicate tosuperposition modulation and how much to dedicate to PAPR reduction. Thehigher the data capacity, the higher the PAPR requiring a largertransmitter to broadcast the super positioned modulation. In FIG. 3, abroadcast transmitter transmitting the described signal is operating atreduced power levels due to the increased PAPR. In order to superimposea 2^(nd) constellation, the first constellation must be moved back to aclean reference point thus reducing the degrees of freedom the PAPRreduction can take, increasing the remaining signal peaks over thenumber of iterations of the reduction algorithm. Note that while cleansuper positioned constellations are shown, it may be feasible tomaintain a small degree of clipping noise within that constellation oruse other methods, such as swapping constellation points, to help withpeak reduction. However, due to the small constellation distance of the2^(nd) super positioned modulation, little margin exists for such ascheme, rendering them less effective while maintaining informationbearing constellations. While FIG. 3 displays IBOC only carriers, it isunderstood that this concept also applies to hybrid FM+IBOC signalconfigurations using hybrid crest factor reduction (Schmid, 2009) orother IBOC or digital radio signal configurations.

The QPSK super positioned modulation method described herein has theadvantage that the 2^(nd) modulator does not need to know theconstellation of the primary modulation. The secondary modulation can bea simple addition to the primary modulation that can be performed eitherin the frequency or time domain. Other modulation or constellationmethods can be applied to this method. FIG. 4 shows a method based onbinary phase shift keying (BPSK) which does require knowledge of theprimary modulation for optimal superposition. Data is assigned to one oftwo constellation points within the first quadrant of the primaryconstellation. The resultant frequency domain constellation point can berotated by 90, 180, or 270 degrees based on the primary modulation. Thebenefit of this method is a greater constellation power leading to amore robust secondary modulation. Secondary constellation points areadjusted to yield the same power spectral density as normal carrierswithout secondary modulation. However, it is possible to assigndiffering power levels to these carriers, but this may violate theprimary modulation signal quality specifications. It may also bepossible to include the primary modulated bit stream in the secondarymodulation process to augment data capacity or forward error correction.

Without wishing to be bound by theory, the method described herein canprovide 50% more data capacity for BPSK or double the broadcast system'sdata capacity for QPSK. However, the method can further comprise acomplete communications system design, including forward errorcorrection (FEC) and other aspects that affect the overall datathroughput. It is expected that because of the reduced constellationpower, which is constrained by the quality specifications (14 dB MER forIBOC, 21 dB MER for FM−DRM), more robust FEC is required for areasonable communications link budget to achieve reasonable coverage,reducing the data capacity.

An added advantage of this method is that the secondary modulation canbe added using a second modulator that is independent from the primarymodulator, as shown in FIG. 5. The two signals are combined within thePAPR reduction component of the overall system, which has knowledge ofhow both modulators are configured. A key benefit of this approach isthat the two modulators can be isolated or air-gapped from a networksecurity point of view. Neither modulator can affect the logical bitstream of the other. This is very important in security sensitiveapplications, such as the electric power grid.

A potential application is to convey power grid information, such ascurrent power rates, to many internet of things (IoT) devices. While theprimary modulation may be a typical digital radio broadcast, with heavymedia integration obtaining content from many sources on the Internetthat cannot be guaranteed to be at the same security level as requiredby the power grid. The secondary modulation can be composed entirelyindependently and can be configured to comply with various securityrequirements; it can be considered air-gapped.

While this method has been described with reference to the IBOC signalused in HD Radio® because of the backward compatibility to the manyexisting receives in the field, the method is equally applicable to manyother digital modulation standards.

While a number of exemplary aspects and embodiments have been discussedabove, those with skill in the art will recognize certain modifications,permutations, additions and sub-combinations thereof. It is thereforeintended that the following appended claims and claims hereafterintroduced are interpreted to include all such modifications,permutations, additions and sub-combinations as are within their truespirit and scope.

REFERENCES

-   Anjali Shastri, Brian William Kroeger. 1999. Method and apparatus    for reducing peak to average power ratio in digital broadcasting    systems. U.S. Pat. No. 6,128,350 Aug. 24, 1999.-   National Radio Systems Committee (NRSC). 2011. Transmission Signal    Quality for IBOC Signals. 2011. 2646s.-   Schmid, Philipp. 2009. An Improved Method of Peak-to-Average Power    Ratio Reduction for FM+IBOC Broadcast Transmission. Halifax: s.n.,    2009.-   Shelswell, Peter. 1992. Digital Signal Transmission System Using    Frequency Division Multiplex. WO 94/06231 Great Britain, Sep.    7, 1992. British Broadcasting Corporation.

1. A method for providing additional bandwidth to receivers that can decode a higher modulation comprising trading a peak-to-average power ratio (PAPR) reduction induced constellation noise of all or a subset of in-band on-channel (IBOC) carriers within an orthogonal frequency division multiplexing (ODFM) waveform with data carrying superposition modulation.
 2. The method of claim 1, wherein the IBOC carrier is Quadrature Phase Shift Keyed (QPSK) modulated.
 3. The method of claim 1, wherein the superposition modulation is hierarchical modulation.
 4. The method of claim 1, wherein a secondary modulation is added using an independent second modulator.
 5. The method of claim 4, wherein the secondary modulation is for security requirements. 